Can Quantum Mechanics Drown Out Your Noisy Neighbours?

HKU Engineering researchers have solved a long-standing dilemma in acoustics: how to silence noise without sealing off fresh air.

In classical acoustics, the mass law dictates a frustrating compromise: the heavier and thicker a barrier, the quieter the room. This principle presents a persistent challenge for architects and designers, who must often choose between blocking noise with dense walls or allowing fresh air to flow through open vents. Conventional attempts to circumvent this limit by tuning thin acoustic metamaterials to a single frequency often suffer from narrow bandwidths, failing to block noise just outside the target range.

To tackle this fundamental limit, an HKU research team led by Professor Nicholas X. Fang of the Department of Mechanical Engineering, looked beyond classical mechanics and into the microscopic quantum world. They adapted the "Baldin-sum-rule" — a powerful theorem originally used to link the electric and magnetic polarizability of atomic nuclei to their absorption of light — and recast it for macroscopic sound waves.

"We discovered that the integral of the extinction cross section is fundamentally locked by the scatterer's static effective mass and stiffness," explains Dr Sichao Qu, the first author on the paper, which was published in the prestigious journal Physical Review Letters.

In plain language: you cannot silence every frequency perfectly, but you can optimally redistribute this finite 'scattering budget' to dramatically broaden the range of blocked frequencies.

The implications reach far beyond the laboratory. Imagine a summer breeze flowing through an open window while city traffic fades to a whisper. Picture hospital intensive-care units free from the relentless drone of ventilators, or airplane cabins where conversation no longer requires raised voices.

"Counterintuitively, our (approach) is remarkably simple, representing the minimal physical elements required to achieve causally optimal scattering," said Professor Fang. "This universal rule provides a practical blueprint for next-generation ventilated silencers that can efficiently block noise without blocking airflow."